1. Field of the Invention
The present invention relates to an image forming apparatus which fixes toner on printing medium by a fixing unit including a first and second radiation heaters which have heating widths different from each other.
2. Description of the Related Art
Conventionally, there has been an image forming apparatus disclosed in JP 2008-203685 A as the above-mentioned image forming apparatus. In JP 2008-203685 A, a heating roller of a fixing unit includes a short heater and a long heater as a first and second radiation heaters. Here, a heating width of the printing medium of the long heater is wider than that of the short heater. The temperature of the fixing unit is controlled so that lighting times of both heaters or use times of them converge on the same value.
In recent years, on/off control of the radiation heater has been performed so that a temperature fluctuation range of the heating roller of the fixing unit is reduced in order to improve image quality. The on/off control to the radiation heater included in the fixing unit has been performed in a short period in many cases. It has been considered so far that an inrush current of the radiation heater has had a small influence on a life of the radiation heater. Under a use condition where on/off control in the short period is often performed, the influence cannot be ignored.
At the time of warm-up or standby, it is necessary to heat a full width of the heating roller, and the long heater having a wide heating width is used. Therefore, the long heater is used at high frequency. Accordingly, when the on/off control is simply performed to the long heater, the number of the damages to the long heater due to an inrush current increases, and the life of the long heater ends earlier.
In many cases, the fixing unit is exchanged by a unit of the fixing unit not the radiation heater. Therefore, even though the short heater has enough time before the end of its life, there is a case where a user needs to exchange the fixing unit due to the end of the life of the long heater. In this way, it is not desirable that times to end the life of both radiation heaters are largely different from each other.